A discharge surface treatment apparatus is an apparatus that generates a pulsed discharge between poles, i.e., between a discharge electrode and a workpiece to form a film made of an electrode material melted by the thermal energy due to the discharge or a substance resultant from the reaction of the electrode material by the thermal energy due to the discharge on the surface of the workpiece. The discharge electrode is obtained by compression-molding of any one of a metal powder, a powder of a metal compound, and a powder of ceramics or a mixture thereof.
The workpiece is, for example, a component used under a high temperature environment, such as a turbine blade in an aircraft gas-turbine engine. For example, for repairing a component of this type used under a high temperature environment, the surface needs to be coated or built-up with a material having strength and lubricity under a high temperature environment, and the discharge surface treatment apparatus is used.
In order to form a dense and thick film by the discharge surface treatment, it is necessary that: the supply of the electrode material to a portion between the poles and subsequent supply of the electrode material to a workpiece surface by a discharge pulse current are adequate; and the supplied electrode material is melted by the discharge pulse current and is strongly bonded to the workpiece surface.
For example, Patent Literature 1 proposes a discharge surface treatment apparatus that can form a dense and relatively thick film by generating a discharge pulse current satisfying the above conditions between the poles. Specifically, Patent Literature 1 illustrates a configuration in which a capacitor is connected in parallel with a discharge electrode and a workpiece as a configuration that generates the discharge pulse current satisfying the above conditions between the poles.
With this configuration, the capacitor is charged by a no-load voltage applied between the poles from a power source. When a discharge occurs: first, a discharge current having a high peak value and short pulse width, which is obtained by adding current from the capacitor to current supplied from the power source, flows between the poles; subsequently, a discharge current having a low current value and long pulse width, which is composed only of current supplied from the power source, flows. In this manner, as the capacitor is connected in parallel with the discharge electrode and the workpiece, a discharge pulse current having a shape of a “high peak value and short pulse width”+“low current value and long pulse width” may be caused to flow between the poles.
In accordance with the discharge pulse current having a shape of a “high peak value and short pulse width”+“low current value and long pulse width”, in the leading current portion having a “high peak value and short pulse width”, the electrode disintegrates and the supply of the electrode material to a portion between the poles is controlled. In the subsequent current portion having a “low current value and long pulse width”, part of the electrode material is melted by the thermal energy due to the discharge and a film is formed on the workpiece surface.
However, at the boundary between the leading pulse current portion and the subsequent pulse current portion, charging of the capacitor is started by a discharge voltage between the poles, therefore, there is a problem in that reversal of the output current direction occurs and a portion having a low output current is generated. Particularly, when the current supplied from the power source is low, a phenomenon may occur in which the discharge current is interrupted at the portion at which reversal of the output current direction occurs. When the discharge current is interrupted in the middle of the process, the necessary discharge thermal energy cannot be obtained and therefore, the electrode material is not melted completely, which results in the formation of the film having a large number of voids and defects.
In order to solve this problem, if the above capacitor is referred to as a first capacitor, Patent Literature 1 proposes a configuration in which a second capacitor is arranged in parallel with the first capacitor and current is supplied from the second capacitor after the occurrence of a discharge, thereby preventing the discharge current from being interrupted due to reversal of the output current direction in the first capacitor.
However, with the configuration in which the first capacitor and the second capacitor are arranged in parallel between the poles, the peak current from the first capacitor and the peak current from the second capacitor are superimposed, therefore, in the leading current portion having a “high peak value and short pulse width” serving to melt the electrode, the current value partially increases. If the current value partially increases in the part of the leading current portion having a “high peak value and short pulse width”, the thermal energy that melts part of the electrode material increases excessively, and the electrode material cannot smoothly move to the film side and thus works to remove the film, thereby forming a film having a large number of voids and defects.